The true wind is felt by a stationary observer, whereas the apparent wind is felt by an observer on a moving boat. It is the apparent wind that is acting upon the sails when the boat is under way. A turning moment is created about the boat's hydrodynamic center, particularly when the boat starts to heel. The hydrodynamic center is simply the center of the hull's side pressures when the boat starts to yaw. The turning moment is torque attempting to turn the boat up into the wind.
To sail a straight course, the turning moment must be counteracted by an equal moment about the hydrodynamic center. This is done by the creation of lift at the rudder, namely weatherhelm, the amount being determined by rudder angle and boat speed. Lift is perpendicular to the water stream, directly proportional to the rudder angle up to the stall position at approximately 14 degrees, for a rudder with 2.5 aspect ratio, and directly proportional to the square of boat speed. Lift decreases when the rudder angle is greater than approximately 14 degrees, and drag, parallel to the water stream, increases, lowers boat speed but helps to pivot the stern about the hydrodynamic center and align the boat with its forward movement.
Boats with wheel steering have a friction brake for securing the helm, thereby fixing the primary rudder angle. Boats with tiller steering can fasten the ends of a line with eye straps to the cockpit side walls and a jam cleat to the top surface of the tiller.
Yawing is at the heart of all steering, whether it be manual or self-steering. It is the rotation of the boat about its hydrodynamic center while moving in the direction of travel. Yawing occurs when the equilibrium between the boat's turning moment and the primary rudder's lift is upset. In the case of manual steering, the boat heading is altered by an increase or decrease of the primary rudder angle, so that the change in lift creates a yawing motion and changes the boat's course.
Yawing can also be the result of wave action or a change in wind speed. Either can change the boat speed. If the boat is equipped with self-steering and has the primary rudder angle fixed, the boat could start to yaw and change course. However, if the self-steering device is doing its job properly, control of the boat's course will be maintained as shown by the vector diagrams of FIGS. 1 through 4. When the boat sails down a wave, there will be an increase in boat speed, and when the boat sails up a wave, there will be a decrease in boat speed. The vector diagrams show the effects of wave action in FIGS. 1 and 2, and the effects of a change in true wind speed in FIGS. 3 and 4. The vectors are identified for increases in boat and wind speeds, however, for decreases, FIGS. 1 through 4 can be given a reverse interpretation.
The present invention is directed to a new type of self-steering device for sailboats, hereafter referred to as Hydraulic Vane Steering HVS! that maintains the angle between boat heading and apparent wind direction on all points of sailing when the boat is under way and registering wind direction. The HVS device may be mounted on the stern of the sailboat in the manner shown hereinafter and includes a water-driven impeller driving a hydraulic vane pump in an impeller drive housing. A clutch connects the windvane to the control shaft that extends vertically through the column tubing to a hydraulic pressure control assembly in the submerged impeller drive housing. The angle of attack and strength of the apparent wind at the windvane can change the oil pressure at internal hydraulic pistons and, by leverage, control the auxiliary rudder angle of attack with the water stream.
Preferably, one HVS device will serve sailboats 25 to 45 feet in length and be located centrally on the stern of boats with an inboard rudder or 12 inches either side of center with an outboard rudder. Sailboats 45 to 60 feet in length can be served by two HVS devices with centers 24 inches apart. The HVS device will handle boat speeds up to 12 knots. The HVS device can be easily mounted on a positive transom, negative transom, vertical transom, round stern, and canoe stern. Various HVS lengths can be utilized for boats having a stern freeboard of 25 to 55 inches.
HVS is provided with an auxiliary rudder lock rod that is normally in a raised position when sailing and lowered to lock the auxiliary rudder aligned with the boat's centerline when at anchor and going astern under power.
To operate the HVS device, head the boat on the desired course with the sails properly trimmed. Release the control line that operates the clutch from the cam cleat and secure the helm. The clutch has become engaged by spring action joining the mating parts together. Self-steering has now taken over. To return to manual steering, disengage the HVS device by pulling and securing the control line in the cam cleat and then free the helm. This will result in disengagement of the clutch and provide free-wheeling to the windvane, allowing it to weathercock. It also allows the auxiliary rudder to align with the water stream and the impeller to turn freely.
Because the HVS auxiliary rudder has a hydrofoil section, shown in FIG. 26, its drag should be little more than the drag from its surface friction. The impeller and impeller drive housing will have some drag that will reduce the boat speed by a small amount, but in a positive way such drag will help the boat to stay on a steady course.
If the primary rudder is fixed by securing the helm so that the primary rudder supplies most of the weatherhelm, thus leaving the self-steering to make only small corrections, an auxiliary rudder can be about one fourth the size of the primary rudder and adequately handle the job. Although the primary rudder is fixed to supply the weatherhelm, the amount of weatherhelm required varies with the strength of the apparent wind. If the wind freshens, more weatherhelm is needed and the auxiliary rudder has to supply more weatherhelm, the auxiliary rudder can do this as the boat turns windward. If the wind lightens, less weatherhelm is needed, and as the boat turns leeward, the auxiliary rudder counteracts some of the weatherhelm of the primary rudder. When the apparent wind returns to the strength it had when the primary rudder was fixed, the boat will return to its original course, and once again the auxiliary rudder will be aligned with the water stream.
All self-steering systems are aerodynamically controlled and basically powered by the apparent wind attacking one side of the windvane. The torque on the windvane shaft varies as a result of the angle of attack and wind strength. Some competitive systems multiply the torque by hydrodynamically using a servo-tab on the trailing edge of the primary rudder or auxiliary rudder. Others use a servo-pendulum to operate the primary rudder. The tab or pendulum is mechanically connected to the windvane, usually with a 1:1 angular movement ratio for a vertical axis windvane. The tab or pendulum is unbalanced so that it tends to self-align with the water stream and provide feedback to the windvane. Feedback is simply torque created by the water stream at the rudder, tab or pendulum that sends its angle information back to the windvane.
When sailing downwind, either reaching or running, there can be rapid acceleration followed by abrupt deceleration which is characteristic of light displacement monohulls as well as multihulls. Rapid acceleration followed by abrupt deceleration can also be caused by wave action with the boat speed increasing and the wind speed decreasing when sailing down a wave or swell, as with a following sea.
Hydrodynamic pressure at the rudder varies as the square of boat speed, and aerodynamic pressure at the windvane varies as the square of wind speed. Assuming a boat is sailing downwind on a dead run with a boat speed of 4 knots, with an apparent wind speed of 8 knots, and a true wind speed of 12 knots, an increase in boat speed of only 1 knot due to wave action would increase the boat speed to 5 knots and decrease the apparent wind speed to 7 knots with the following results: ##EQU1##
In the preceding situation when the primary rudder is controlled by a servo-pendulum as with competitive self-steering devices, the rudder and pendulum attack angle with the water stream could decrease and the windvane attack angle with the apparent wind could increase by the boat turning windward. However, the latter may not prevent the loss of windvane control of the boat's course because the feedback from the rudder and pendulum could overpower the windvane. The hydrodynamic force is 830 times more effective than the aerodynamic force. Dynamic pressure and lift are proportional to the density of the fluid, both liquid and gas.
Up to now, no commercially available self-steering system can really do the job well. The most common problems are lack of power and sensitivity, and sometimes over-steering as well. The smaller the windvane, the less is the inertia and the quicker is the reponse to being off course and returning to course. The HVS windvane is relatively small, but very sensitive to changes in wind direction and strength because it need only transmit a very small amount of force to the control plungers at the control orifices of the hydraulic control assembly in the submerged enclosure. For example, only 1 ounce of force need be applied by the wind at the center of pressure of the windvane to create a lift of 107 pounds at the center of pressure of the auxiliary rudder, which is a power ratio of 1712:1, which is calculated with the following dimensions and calculations:
Windvane center of pressure arm CP!=8.75 PA1 Control rocker arm=1 PA1 Control orifice area=0.00385 0.070 dia.! PA1 Piston area=0.3712 sq. in. 0.6875 dia.! PA1 Rocker shaft arm=0.812 PA1 Aux. rudder control arm=2.50 PA1 Aux. rudder arm=3.125 PA1 Aux. rudder center of pressure=0.50 ##EQU2## PA1 1. High power ratio between auxiliary rudder and windvane. PA1 2. Little windvane movement needed to change oil pressure. PA1 3. Elimination of feedback from aux. rudder to windvane. PA1 4. Control of boat's course on all points of sailing. PA1 5. Course correction that is proportional to the degree of deviation from course.
Obviously, the ratio is dependent on a number of variables which can be modified to suit different purposes, but the main point is that very little force at the windvane results in relatively very high forces acting upon the auxiliary rudder, so as to effectively move such when needed.
When sailing downwind, with the boat speed increasing due to wave action, as with a following sea, and apparent wind strength dropping, HVS will maintain control of the boat's course because of the high power ratio between auxiliary rudder and windvane. The increased hydrodynamic pressure on the HVS auxiliary rudder cannot cause overpowering feedback to the windvane, as it can with competitive equipment, because the auxiliary rudder is not directly connected to the windvane. The angle of the HVS auxiliary rudder would decrease, but its lift would remain whatever the windvane calls for.
The water turbulence from the primary rudder does not prohibit locating the HVS device directly aft and close to the primary rudder. HVS auxiliary rudder lift is determined by the HVS windvane and is unaffected by the water stream turbulence. This is not true for current self-steering auxiliary rudders because they are mechanically connected to their windvane, usually through their pendulum or trim tab.
With competitive equipment, when a sailboat is driven off course, the number of degrees off course before starting the return to course equals the sum of the changes in the windvane angle of attack and the angle that the windvane turns relative to the boat in order to rotate the servo-pendulum or trim-tab with the relationship usually being 1:1 for a vertical axis windvane. With HVS, very little windvane movement is needed to move the pressure control plungers. The apparent wind angle of attack at the windvane is almost all that is needed to bring the boat back on course. This results in less degrees off course and a speedier return to course.
With competitive self-steering equipment on downwind courses, the diminished wind strength at the windvane and overpowering feedback from rudder, tab, or pendulum to the windvane, when boat speed increases due to wave action, can cause failure of the system to control the boat's course. With HVS, the use of hydraulics in the manner used by HVS makes possible the following advantages:
Autopilots are distinguished from self-steering mechanisms in that they require electrical energy and probably should be used only when the sailboat is under power, with the sails furled, or when there is very little wind. Accordingly, autopilots are not believed pertinent to the current discussion of self-steering mechanisms for sailboats.
Presently, the most popular type of self-steering equipment is the servo-pendulum, and examples of such devices are marketed by the companies Sailomat USA, Fleming Marine, and Scanmar Marine (Monitor). When these devices are engaged to keep the boat on course, the apparent wind strikes one side of the windvane and through gears the windvanes's motion causes the servo-pendulum to rotate. Water pressure against the rotated pendulum causes it to swing up to windward. Through blocks, the swinging pendulum pulls lines attached to the tiller or drum on the steering wheel. If the boat heads off course windward, or the apparent wind speed increases, the windvane will cause the pendulum to swing up an additional amount towards the water surface, always on the windward side. This rotation is added to the boat's angle of heel. Here it begins to emerge from the water, its lift is greatly reduced just by being close to the surface, and worse, it can ventilate or draw a bubble of air down along its upper (suction) side, so its lift falls drastically, at a time when more lift is needed.
Servo-pendulum self-steering uses the primary rudder for self-steering. The maximum rudder lift obtainable varies with the rudder aspect ratio, which is rudder depth in water divided by rudder width, for a rectangular rudder. For example, a rectangular rudder with an aspect ratio of 2.5, the maximum rudder lift obtainable is when the rudder angle of attack is approximately 14 degrees. When the rudder angle is 8 degrees, for example, providing the weatherhelm needed to keep the boat on course, there remains only 6 degrees to the stall angle for the self-steering system to use. Also, the amount of power that the self-steering system must generate to operate the primary rudder could be considerable.
With Hydraulic Vane Steering, when the primary rudder is fixed by securing the helm so that it supplies most of the weatherhelm needed, leaving the self-steering to make only small corrections, the HVS auxiliary rudder can be about one fourth the size of the primary rudder, and adequately handle the job. Considerably less power is needed to operate the auxiliary rudder, and there is the full 14 degrees each side of center for the auxiliary rudder to use. By keeping the auxiliary rudder away from stall, its ability to respond with large turning moments in either direction is assured.
The Sailomat, Fleming, and Monitor have control lines and blocks cluttering the cockpit. The control lines lead from the pendulum to the tiller or wheel drum and should be slack free. However, friction should be avoided anywhere in the system. It is possible to lash the control lines up so tightly that the effort to move the rudder is actually increased. With Hydraulic Vane Steering, the auxiliary rudder movement is controlled directly by a roller and lever at the lower end of the auxiliary rudder.
The apparent wind strength and the windvane angle of attack determine the windvane lift. In contrast to prior art systems, with HVS, the windvane lift controls the hydraulic pressure in two built-in hydraulic cylinders in the submerged enclosure impeller drive housing! located directly beneath the auxiliary rudder. The water-driven impeller provides the torque needed by the vane pump in the submerged enclosure to create the hydraulic pressure. The movement of the pistons in the cylinders is transmitted from the submerged enclosure through leverage and a roller to the lower end of the auxiliary rudder, thereby controlling the auxiliary rudder angle of attack with the water stream and creating the lift needed to return the boat to course. It is believed that the foregoing method of sailboat self-steering is thus unique.
Other features and advantages of the invention shall become apparent as the description thereof proceeds, when considered in connection with the accompanying illustrative drawings.